In order to manufacture a desired semiconductor unit, various thermal processes including an oxidation process, a diffusion process, a CVD process, an annealing process or the like are carried out to an object to be processed such as a semiconductor wafer. As a thermal processing unit for conducting the processes, a vertical thermal processing unit is used wherein a large number of wafers are thermally processed at a time.
The vertical thermal processing unit has a processing container made of quartz, which forms a vertical thermal processing furnace. A furnace opening that opens at a lower end of the processing container can be opened and closed by a lid. A boat (holding tool) made of quartz that holds a large number of wafers in a tier-like manner is supported on the lid via a heat insulating cylinder made of quartz. In addition, around the processing container, a heater is arranged, which consists of a cylindrical heat insulating material and a heating resistance wire shaped into a spiral or the like provided in the cylindrical heat insulating material.
This type of conventional thermal processing unit is designed in such a manner that a thermal process is conducted in a relatively high temperature zone of, for example, about 900° C. to 1200° C. In addition, in this type of conventional thermal processing unit, after the thermal process, it is difficult for the temperature of the wafer to be lowered. Thus, in order to lower the temperature of the wafer, a structure that blows a cooling wind at an outside of the processing container has been proposed (for example, JP Laid-Open Publication No. 2000-100812).
Herein, as a thermal processing step, not in the high temperature zone, the wafer may be thermally processed in a relatively low temperature zone of, for example, about 50° C. to 600° C. For example, in order to reduce wire capacitance, if an organic film such as a resin having a small dielectric constant is used as an interlayer insulating film, the organic film may have to be hardened (vitrified) in a low temperature of about 40° C. to 600° C. It is possible to conduct a thermal process of a low temperature zone in a thermal processing unit designed for a high temperature.
However, in the conventional thermal processing unit, the processing container is made of quartz, and thus thermal capacity thereof is so large. In addition, the insulating material of the heater surrounds the processing container. Thus, even if the wafer is processed in a low temperature, it takes a long time to lower the wafer temperature to a handling temperature such as a room temperature. For example, when the wafer is naturally cooled to a room temperature after a thermal process, the temperature-fall rate (cooling speed) was less than 1° C./min, which is low. That is, the temperature-fall property of the wafer in a low temperature zone was inferior. This phenomenon was not improved so much by means of a unit that blows a cooling wind at a side wall of the processing container.